1. Field of the Invention
The present invention relates to a fixing device mounted on image forming apparatuses such as a copying machine, a printer, and a facsimile, and, more particularly to a fixing device for an image forming apparatus that performs warming-up at high speed.
2. Description of the Related Art
As a fixing device of an induction heating system used in image forming apparatuses of an electrophotographic system such as a copying machine and a printer, there is a device that quickens a rise in temperature of the fixing device to realize a reduction in a warming-up time. For example, JP-A-2002-295452 discloses a heating device that causes a metal sleeve having a small heat capacity on an outer circumference of a heat roller to generate heat using an induction coil and reduces a warming-up time.
However, if the heat capacity of the heat roller is reduced using the metal sleeve in order to quicken a temperature rise in the heat roller as in the heating device in the past, it is difficult for the heat roller to keep fixing temperature when image formation is continuously performed. When the temperature of the heat roller falls, the heat roller has to be heated to fixable temperature again. Therefore, fixing of images takes time, an increase in speed of image formation is not realized, and it is likely that fixing performance is adversely affected.
Therefore, as the fixing device of the induction heating system, there is a demand for development of a fixing device for an image forming apparatus that can keep fixing temperature even in continuous image formation without damaging a high-speed temperature rise in warming-up and obtain a high-quality fixed image at high speed.